Advanced polymer wood composite

ABSTRACT

A composition in the form of pellets comprising a thermoplastic and wood fiber composite material suitable for forming structural members as a replacement for wood in the manufacture of doors and windows. The composite has less than about 10 wt % water based on pellet weight and a Young&#39;s modulus of at least about 500,000. Structural members are typically formed from the composite in an extrusion or an injection molding process.

This application is a continuation of U.S. Ser. No. 08/543,959 filedOct. 17, 1995, now U.S. Pat. No. 5,827,607; which is a continuation ofU.S. Ser. No. 08/224,396 filed Apr. 7, 1994, now abandoned; which is acontinuation of U.S. Ser. No. 07/938,364 filed Aug. 31, 1992, nowabandoned.

FIELD OF THE INVENTION

This invention relates to composite thermoplastic materials used for thefabrication of structural members. Such members an be any structuralunit, preferably they are for use in windows and doors for residentialand commercial architecture. These composite materials include anoptional, intentional recycle of by-product streams, from window anddoor manufacture, of polymer, adhesive, paint, etc. More particularly,the invention relates to an improved composite material adapted toextrusion or injection molding processes for forming structural membersthat have improved properties when used in windows and doors. Thecomposite materials of the invention can be made to manufacturestructural components such as rails, jambs, stiles, sills, tracks, stopand sash, nonstructural trim elements such as grid, cove, bead, quarterround, etc.

BACKGROUND OF THE INVENTION

Conventional window and door manufacture has commonly used wood andmetal components in forming structural members. Commonly, residentialwindows are manufactured from milled wood products that are assembledwith glass to form typically double hung or casement units. Wood windowswhile structurally sound, useful and well adapted for use in manyresidential installations, can deteriorate under certain circumstances.Wood windows also require painting and other periodic maintenance.Wooden windows also suffer from cost problems related to theavailability of suitable wood for construction. Clear wood products areslowly becoming more scarce and are becoming more expensive as demandincreases. Metal components are often combined with glass and formedinto single unit sliding windows. Metal windows typically suffer fromsubstantial energy loss during winter months.

Extruded thermoplastic materials have been used in window and doormanufacture. Filled and unfilled thermoplastics have been extruded intouseful seals, trim, weatherstripping, coatings and other windowconstruction components. Thermoplastic materials such as polyvinylchloride have been combined with wood members in manufacturingPERMASHIELD® brand windows manufactured by Andersen Corporation for manyyears. The technology disclosed in Zanini, U.S. Pat. Nos. 2,926,729 and3,432,883, have been utilized in the manufacturing of plastic coatingsor envelopes on wooden or other structural members. Generally, thecladding or coating technology used in making PERMASHIELD® windowsinvolves extruding a thin polyvinyl chloride coating or envelopesurrounding a wooden structural member.

Polyvinyl chloride has been combined with wood to make extrudedmaterials. However, such materials have not successfully been used inthe form of a structural member that is a direct replacement for wood.Common thermoplastic composite materials cannot provide similar thermaland structural properties to wood. These extruded materials fail to havesufficient modulus, compressive strength, coefficient of thermalexpansion that matches wood to produce a direct replacement material.Further, many prior art extruded composites must be milled afterextrusion to a final useful shape. Typical commodity plastics haveachieved a modulus no greater than about 500,000. One class ofcomposite, a polyvinyl chloride/wood flour material, poses the addedproblem that wood dust, which can accumulate during manufacture, tendsto be explosive at certain concentrations of wood flour in the air. Mostcommonly, poly vinyl chloride, polystyrene and polyethylenethermoplastics have been used in such products.

Accordingly, a substantial need exists for a composite material that canbe made of polymer and wood fiber with an optional, intentional recycleof a waste stream. A further need exists for a composite material thatcan be extruded into a shape that is a direct substitute for theequivalent milled shape in a wooden or metal structural member. Thisneed requires a coefficient of thermal expansion that approximates wood,a material that can be extruded into reproducible stable dimensions, ahigh compressive strength, a low thermal transmission rate, an improvedresistance to insect attack and rot while in use and a hardness andrigidity that permits sawing, milling, and fastening retentioncomparable to wood members. Further, companies manufacturing window anddoor products have become significantly sensitive to waste streamsproduced in the manufacture of such products. Substantial quantities ofwood waste including wood trim pieces, sawdust, wood millingby-products; recycled thermoplastic including recycled polyvinylchloride, has caused significant expense to window manufacturers.Commonly, these materials are either burned for their heat value inelectrical generation or are shipped to qualified landfills fordisposal. Such waste streams are contaminated with substantialproportions of hot melt and solvent-based adhesives, waste thermoplasticsuch as polyvinyl chloride, paint, preservatives, and other organicmaterials. A substantial need exists to find a productiveenvironmentally compatible use for such waste streams to avoid returningthe materials into the environment in an environmentally harmful way.

BRIEF DISCUSSION OF THE INVENTION

We have found that the problems relating to forming a substitute forwood and metal structural members and the problems relating to therecycle of waste streams in window manufacture can be solved by forminga polymer/wood fiber composite material into window and door structuralmembers. The polyvinyl chloride can be combined with wood fiber andwaste materials to form a polyvinyl chloride/wood fiber composite,preferably in the form of a pellet. The wood fiber comprises the sawdustor milling byproduct waste stream from milling wooden members in windowmanufacture and can be contaminated with substantial proportions of hotmelt adhesive, paint, solvent or adhesive components, preservatives,polyvinyl chloride recycle pigment, plasticizers, etc. We have foundthat the PVC and wood fiber composite can be manufactured intoacceptable substitutes for wooden members if the PVC and wood materialcontains less than about 10 wt-%, preferably less than 3.5% water basedon pellet weight. The compositions can achieve in a final product highmodulus, high compressive strength, reproducible, stable dimensions anda superior modulus of elasticity. We have also found that the successfulmanufacture of structural members for windows and doors requires thepreliminary manufacture of the polyvinyl chloride wood fiber compositein the form of a pellet wherein the materials are intimately mixed andcontacted in forming the pellet prior to the extrusion of the membersfrom the pellet material. We have found that the intimate mixing of thepolyvinyl chloride, wood fiber, and waste in the manufacture of thepellet process with associated control of moisture content produces apelletized product that is uniquely adapted to the extrusion manufactureof PVC/wood fiber components and achieves the manufacture of a usefulwood replacement product.

DETAILED DESCRIPTION OF THE INVENTION

The invention relates to the use of polyvinyl chloride and wood fibercomposite materials with a controlled water content in the form of apelletized material wherein the wood fiber is intimately contacted andwetted by the organic materials. The intimate contact and wettingbetween the components in the pelletizing process ensures high qualityphysical properties in the extruded composite materials aftermanufacture.

Pellet

The polyvinyl chloride and wood fiber can be combined and formed into apellet using a thermoplastic extrusion processes. Wood fiber can beintroduced into pellet making process in a number of sizes. We believethat the wood fiber should have a minimum size of length and width of atleast 1 mm because wood flour tends to be explosive at certain wood toair ratios. Further, wood fiber of appropriate size of a aspect ratiogreater than 1 tends to increase the physical properties of the extrudedstructural member. However, useful structural members can be made with afiber of very large size. Fibers that are up to 3 cm in length and 0.5cm in thickness can be used as input to the pellet or linear extrudatemanufacturing process. However, particles of this size do not producehighest quality structural members or maximized structural strength. Thebest appearing product with maximized structural properties aremanufactured within a range of particle size as set forth below.Further, large particle wood fiber an be reduced in size by grinding orother similar processes that produce a fiber similar to sawdust havingthe stated dimensions and aspect ratio. One further advantage-ofmanufacturing sawdust of the desired size is that the material can bepre-dried before introduction into the pellet or linear extrudatemanufacturing process. Further, the wood fiber can be pre-pelletizedinto pellets of wood fiber with small amounts of binder if necessary.

During the pelletizing process for the composite pellet, the polyvinylchloride and wood fiber are intimately contacted at high temperaturesand pressures to insure that the wood fiber and polymeric material arewetted, mixed and extruded in a form such that the polymer material, ona microscopic basis, coats and flows into the pores, cavity, etc., ofthe fibers. The fibers are preferably substantially oriented by theextrusion process in the extrusion direction. Such substantialorientation causes overlapping of adjacent parallel fibers and polymericcoating of the oriented fibers resulting a material useful formanufacture of improved structural members with improved physicalproperties. The degree of orientation is about 20%, preferably 30% aboverandom orientation which is about 45 to 50%. The structural members havesubstantially increased strength and tensile modulus with a coefficientof thermal expansion and a modulus of elasticity that is optimized forwindow and doors. The properties are a useful compromise between wood,aluminum and neat polymer.

Moisture control is an important element of manufacturing a usefullinear extrudate or pellet. Depending on the equipment used andprocessing conditions, control of the water content of the linearextrudate or pellet can be important in forming a successful structuralmember substantially free of internal voids or surface blemishes. Theconcentration of water present in the sawdust during the formation ofpellet or linear extrudate when heated can flash from the surface of thenewly extruded structural member and can come as a result of a rapidvolatilization, form a steam bubble deep in the interior of the extrudedmember which can pass from the interior through the hot thermoplasticextrudate leaving a substantial flaw. In a similar fashion, surfacewater can bubble and leave cracks, bubbles or other surface flaws in theextruded member.

Trees when cut depending on relative humidity and season can containfrom 30 to 300 wt-% water based on fiber content. After rough cuttingand finishing into sized lumber, seasoned wood can have a water contentof from 20 to 30 wt-% based on fiber content. Kiln dried sized lumbercut to length can have a water content typically in the range of 8 to12%, commonly 8 to 10 wt-% based on fiber. Some wood source, such aspoplar or aspen, can have increased moisture content while some hardwoods can have reduced water content.

Because of the variation in water content of wood fiber source and thesensitivity of extrudate to water content control of water to a level ofless than about 8 wt-% in the pellet based on pellet weight isimportant. Structural members extruded in non-vented extrusion process,the pellet should be as dry as possible and have a water content between0.01 and 5%, preferably less than 3.5 wt-%. When using vented equipmentin manufacturing the extruded linear member, a water content of lessthan about 8 wt-% can be tolerated if processing conditions are suchthat vented extrusion equipment can dry the thermoplastic material priorto the final formation of the structural member of the extrusion head.

The pellets or linear extrudate of the invention are made by extrusionof the polyvinyl chloride and wood fiber composite through an extrusiondie resulting in a linear extrudate that can be cut into a pellet shape.The pellet cross-section can be any arbitrary shape depending on theextrusion die geometry. However, we have found that a regular geometriccross-sectional shape can be useful. Such regular cross-sectional shapesinclude a triangle, a square, a rectangle, a hexagonal, an oval, acircle, etc. The preferred shape of the pellet is a regular cylinderhaving a roughly circular or somewhat oval cross-section. The pelletvolume is preferably greater than about 12 mm³. The preferred pellet isa right circular cylinder, the preferred radius of the cylinder is atleast 1.5 mm with a length of at least 1 mm. Preferably, the pellet hasa radius of about 1 to 5 mm and a length of about 1 to 10 mm. Mostpreferably, the cylinder has a radius of 2.3 to 2.6 mm, a length of 2.4to 4.7 mm, a volume of 40 to 100 mm³, a weight of 40 to 130 mg and abulk density of about 0.2 to 0.8 gm/mm³.

We have found that the interaction, on a microscopic level, between thepolymer mass and the wood fiber is an important element of theinvention. We have found that the physical properties of an extrudedmember are improved when the polymer melt during extrusion of the pelletor linear member thoroughly wets and penetrates the wood fiberparticles. The thermoplastic material comprises an exterior continuousorganic polymer phase with the wood particle dispersed as adiscontinuous phase in the continuous polymer phase. The material duringmixing and extrusion obtains an aspect ratio of at least 1.1 andpreferably between 2 and 4, optimizes orientation such as at least 20wt-%, preferably 30% of the fibers are oriented in an extruder directionand are thoroughly mixed and wetted by the polymer such that allexterior surfaces of the wood fiber are in contact with the polymermaterial. This means, that any pore, crevice, crack, passage way,indentation, etc., is fully filled by thermoplastic material. Suchpenetration as attained by ensuring that the viscosity of the polymermelt is reduced by operations at elevated temperature and the use ofsufficient pressure to force the polymer into the available internalpores, cracks and crevices in and on the surface of the wood fiber.

During the pellet or linear extrudate manufacture, substantial work isdone in providing a uniform dispersion of the wood into the polymermaterial. Such work produces substantial orientation which when extrudedinto a final structural member, permits the orientation of the fibers inthe structural member to be increased in the extruder directionresulting in improved structural properties.

The pellet dimensions are selected for both convenience in manufacturingand in optimizing the final properties of the extruded materials. Apellet is with dimensions substantially less than the dimensions setforth above are difficult to extrude, pelletize and handle in storage.Pellets larger than the range recited are difficult to introduce intoextrusion or injection molding equipment, and are different to melt andform into a finished structural member.

Polyvinyl Chloride Homopolymer, Copolymers and Polymeric Alloys

Polyvinyl chloride is a common commodity thermoplastic polymer. Vinylchloride monomer is made from a variety of different processes such asthe reaction of acetylene and hydrogen chloride and the directchlorination of ethylene. Polyvinyl chloride is typically manufacturedby the free radical polymerization of vinyl chloride resulting in auseful thermoplastic polymer. After polymerization, polyvinyl chlorideis commonly combined with thermal stabilizers, lubricants, plasticizers,organic and inorganic pigments, fillers, biocides, processing aids,flame retardants and other commonly available additive materials.Polyvinyl chloride can also be combined with other vinyl monomers in themanufacture of polyvinyl chloride copolymers. Such copolymers can belinear copolymers, branched copolymers, graft copolymers, randomcopolymers, regular repeating copolymers, block copolymers, etc.Monomers that can be combined with vinyl chloride to form vinyl chloridecopolymers include a acrylonitrile; alpha-olefins such as ethylene,propylene, etc.; chlorinated monomers such as vinylidene dichloride,acrylate momoners such as acrylic acid, methylacrylate,methylmethacrylate, acrylamide, hydroxyethyl acrylate, and others;styrenic monomers such as styrene, alphamethyl styrene, vinyl toluene,etc.; vinyl acetate; and other commonly available ethylenicallyunsaturated monomer compositions.

Such monomers can be used in an amount of up to about 50 mol-%, thebalance being vinyl chloride. Polymer blends or polymer alloys can beuseful in manufacturing the pellet or linear extrudate of the invention.Such alloys typically comprise two miscible polymers blended to form auniform composition. Scientific and commercial progress in the area ofpolymer blends has lead to the realization that important physicalproperty improvements can be made not by developing new polymer materialbut by forming miscible polymer blends or alloys. A polymer alloy atequilibrium comprises a mixture of two amorphous polymers existing as asingle phase of intimately mixed segments of the two macro molecularcomponents. Miscible amorphous polymers form glasses upon sufficientcooling and a homogeneous or miscible polymer blend exhibits a single,composition dependent glass transition temperature (T_(g)). Immiscibleor non-alloyed blend of polymers typically displays two or more glasstransition temperatures associated with immiscible polymer phases. Inthe simplest cases, the properties of polymer alloys reflect acomposition weighted average of properties possessed by the components.In general, however, the property dependence on composition varies in acomplex way with a particular property, the nature of the components(glassy, rubbery or semi-crystalline), the thermodynamic state of theblend, and its mechanical state whether molecules and phases areoriented. Polyvinyl chloride forms a number of known polymer alloysincluding, for example, polyvinyl chloride/nitrile rubber; polyvinylchloride and related chlorinated copolymers and terpolymers of polyvinylchloride or vinylidine dichloride; polyvinyl chloride/alphamethylstyrene-acrylonitrile copolymer blends; polyvinyl chloride/polyethylene;polyvinyl chloride/chlorinated polyethylene and others.

The primary requirement for the substantially thermoplastic polymericmaterial is that it retain sufficient thermoplastic properties to permitmelt blending with wood fiber, permit formation of linear extrudatepellets, and to permit the composition material or pellet to be extrudedor injection molded in a thermoplastic process forming the rigidstructural member. Polyvinyl chloride homopolymers copolymers andpolymer alloys are available from a number of manufacturers includingB.F. Goodrich, Vista, Air Products, Occidental Chemicals, etc. Preferredpolyvinyl chloride materials are polyvinyl chloride homopolymer having amolecular weight of about 90,000±50,000, most preferably about88,000±10,000.

Wood Fiber

Wood fiber, in terms of abundance and suitability can be derived fromeither soft woods or evergreens or from hard woods commonly known asbroad leaf deciduous trees. Soft woods are generally preferred for fibermanufacture because the resulting fibers are longer, contain highpercentages of lignin and lower percentages of hemicellulose than hardwoods. While soft wood is the primary source of fiber for the invention,additional fiber make-up can be derived from a number of secondary orfiber reclaim sources including bamboo, rice, sugar cane, and recycledfibers from newspapers, boxes, computer printouts, etc.

However, the primary source for wood fiber of this invention comprisesthe wood fiber by-product of sawing or milling soft woods commonly knownas sawdust or milling tailings. Such wood fiber has a regularreproducible shape and aspect ratio. The fibers based on a randomselection of about 100 fibers are commonly at least 3 mm in length, 1 mmin thickness and commonly have an aspect ratio of at least about 1.8.Preferably, the fibers are 1 to 10 mm in length, 0.3 to 1.5 mm inthickness with an aspect ratio between 2 and 7, preferably 2.5 to 6.0.The preferred fiber for use in this invention are fibers derived fromprocesses common in the manufacture of windows and doors. Wooden membersare commonly ripped or sawed to size in a cross grain direction to formappropriate lengths and widths of wood materials. The by-product of suchsawing operations is a substantial quantity of sawdust. In shaping aregular shaped piece of wood into a useful milled shape, wood iscommonly passed through machines which selectively removes wood from thepiece leaving the useful shape. Such milling operations producessubstantial quantities of sawdust or mill tailing by-products. Lastly,when shaped materials are cut to size and mitered joints, butt joints,overlapping joints, mortise and tenon joints are manufactured frompre-shaped wooden members, substantial waste trim is produced. Suchlarge trim pieces are commonly cut and machined to convert the largerobjects into wood fiber having dimensions approximating sawdust or milltailing dimensions. The wood fiber sources of the invention can beblended regardless of particle size and used to make the composite. Thefiber stream can be pre-sized to a preferred range or can be sized afterblending. Further, the fiber can be pre-pelletized before use incomposite manufacture.

Such sawdust material can contain substantial proportions of wastestream by-products. Such by-products include waste polyvinyl chloride orother polymer materials that have been used as coating, cladding orenvelope on wooden members; recycled structural members made fromthermoplastic materials; polymeric materials from coatings; adhesivecomponents in the form of hot melt adhesives, solvent based adhesives,powdered adhesives, etc.; paints including water based paints, alkydpaints, epoxy paints, etc.; preservatives, anti-fungal agents,anti-bacterial agents, insecticides, etc., and other waste streamscommon in the manufacture of wooden doors and windows. The total wastestream content of the wood fiber materials is commonly less than 25 wt-%of the total wood fiber input into the polyvinyl chloride wood fiberproduct. Of the total waste recycle, approximately 10 wt-% of that cancomprise a vinyl polymer commonly polyvinyl chloride. Commonly, theintentional recycle ranges from about 1 to about 25 wt-%, preferablyabout 2 to about 20 wt-%, most commonly from about 3 to about 15 wt-% ofcontaminants based on the sawdust.

Composition and Pellet Manufacture

In the manufacture of the composition and pellet of the invention, themanufacture and procedure requires two important steps. A first blendingstep and a second pelletizing step.

During the blending step, the polymer and wood fiber are intimatelymixed by high shear mixing components with recycled material to form apolymer wood composite wherein the polymer mixture comprises acontinuous organic phase and the wood fiber with the recycled materialsforms a discontinuous phase suspended or dispersed throughout thepolymer phase. The manufacture of the dispersed fiber phase within acontinuous polymer phase requires substantial mechanical input. Suchinput can be achieved using a variety of mixing means includingpreferably extruder mechanisms wherein the materials are mixed underconditions of high shear until the appropriate degree of wetting andintimate contact is achieved. After the materials are fully mixed, themoisture content can be controlled at a moisture removal station. Theheated composite is exposed to atmospheric pressure or reduced pressureat elevated temperature for a sufficient period of time to removemoisture resulting in a final moisture content of about 8 wt-% or less.Lastly, the polymer fiber is aligned and extruded into a useful form.

The preferred equipment for mixing and extruding the composition andwood pellet of the invention is an industrial extruder device. Suchextruders can be obtained from a variety of manufacturers includingCincinnati Millicron, etc.

The materials feed to the extruder can comprise from about 30 to 50 wt-%of sawdust including recycled impurity along with from about 50 to 70wt-% of polyvinyl chloride polymer compositions. Preferably, about 35 to45 wt-% wood fiber or sawdust is combined with 65 to 55 wt-% polyvinylchloride homopolymer. The polyvinyl chloride feed is commonly in a smallparticulate size which can take the form of flake, pellet, powder, etc.Any polymer form can be used such that the polymer can be dry mixed withthe sawdust to result in a substantially uniform pre-mix. The wood fiberor sawdust input can be derived from a number of plant locationsincluding the sawdust resulting from rip or cross grain sawing, millingof wood products or the intentional commuting or fiber manufacture fromwaste wood scrap. Such materials can be used directly from theoperations resulting in the wood fiber by-product or the by-products canbe blended to form a blended product. Further, any wood fiber materialalone, or in combination with other wood fiber materials, can be blendedwith waste stream by-product from the manufacturer of wood windows asdiscussed above. The wood fiber or sawdust can be combined with otherfibers and recycled in commonly available particulate handlingequipment.

Polymer and wood fiber are then dry blended in appropriate proportionsprior to introduction into blending equipment. Such blending steps canoccur in separate powder handling equipment or the polymer fiber streamscan be simultaneously introduced into the mixing station at appropriatefeed ratios to ensure appropriate product composition.

In a preferred mode, the wood fiber is placed in a hopper, controlled byweight or by volume, to meter the sawdust at a desired volume while thepolymer is introduced into a similar hopper have a volumetric meteringinput system. The volumes are adjusted to ensure that the compositematerial contains appropriate proportions on a weight basis of polymerand wood fiber. The fibers are introduced into a twin screw extrusiondevice. The extrusion device has a mixing section, a transport sectionand melt section. Each section has a desired heat profile resulting in auseful product. The materials are introduced into the extruder at a rateof about 600 to about 1000 pounds of material per hour and are initiallyheated to a temperature of about 215-225° C. In the intake section, thestage is maintained at about 215° C. to 225° C. In the mixing section,the temperature of the twin screw mixing stage is staged beginning at atemperature of about 205-215° C. leading to a final temperature in themelt section of about 195-205° C. at spaced stages. Once the materialleaves the blending stage, it is introduced into a three stage extruderwith a temperature in the initial section of 185-195° C. wherein themixed thermoplastic stream is divided into a number of cylindricalstreams through a head section and extruded in a final zone of 195-200°C. Such head sections can contain a circular distribution (6-8"diameter) of 10 to 500 or more, preferably 20 to 250 orifices having across-sectional shape leading to the production of a regular cylindricalpellet. As the material is extruded from the head it is cut with adouble-ended knife blade at a rotational speed of about 100 to 400 rpmresulting in the desired pellet length.

The following examples were performed to further illustrate theinvention that is explained in detail above. The following informationillustrates the typical production conditions and compositions and thetensile modulus of a structural member made from the pellet. Thefollowing examples and data contain a best mode.

EXAMPLE 1

A Cincinnati millicron extruder with an HP barrel, Cincinnati pelletizerscrews, an AEG K-20 pelletizing head with 260 holes, each hole having adiameter of about 0.0200 inches was used to make the pellet. The inputto the pelletizer comprised approximately 60 wt-% polymer and 40 wt-%sawdust. The polymer material comprises a thermoplastic mixture ofapproximately 100 parts of polyvinyl chloride homopolymer (in. weight of88,000±2000), about 15 parts titanium dioxide, about 2 parts ethylenebis-stearamide wax lubricant, about 1.5 parts calcium stearate, about7.5 parts Rohm & Haas 820-T acrylic resin impact modifier/process aidand about 2 parts of dimethyl tin thioglycolate. The sawdust comprises awood fiber particle containing about 5 wt-% recycled polyvinyl chloridehaving a composition substantially identical to that recited above.

The initial melt temperature in the extruder was maintained between 350°C. and 400° C. The pelletizer was operated at a polar vinylchloride-sawdust composite combined through put of 800 pounds per hour.In the initial extruder feed zone, the barrel temperature was maintainedbetween 215-225° C. In the intake zone, the barrel was maintained at215-225° C., in the compression zone the temperature was maintained atbetween 205-215° C. and in the melt zone the temperature was maintainedat 195-205° C. The die was divided into three zones, the first zone at185-195° C., the second die zone at 185-195° C. and in the final diezone at 195-205° C. The pelletizing head was operated at a settingproviding 100 to 300 rpm resulting in a pellet with a diameter of 5 mmand a length as shown in the following Table.

                                      TABLE I                                     __________________________________________________________________________    PELLETIZER RESULTS                                                                  Pelletizer                                                                          Pellet        Sawdust                                                                            Pellet                                                                             Pellet Bulk                                                                         Profile                                                                           Tensile                         PVC/Wood                                                                            Melt  Length                                                                            Profile Melt                                                                        Profile                                                                           Moisture                                                                           Moisture                                                                           Density                                                                             Density                                                                           Modulus                         Fiber (Temp, ° F.)                                                                 (in.)                                                                             (Temp. ° F.)                                                                 Visc.                                                                             (%)  (%)  (g/cc)                                                                              (g/cc)                                                                            (psi)                           __________________________________________________________________________    60/40       0.233                                                                             366   2580                                                                              4.71, 4.83                                                                         0.96 0.546 (.006)                                                                        1.426                                                                             990600                                          365   2755                                                    60/40       0.233                                                                             362   2452                                                                              4.71, 4.83                                                                         0.96 0.546 (.006)                              70/30 375   0.080                                                                             375   2274                                                                              5.28 1.54 0.454 (.007)                                                                        1.43                                                                              733300                          70/30 375   0.080                                                                             376   2299                                                                              5.28 1.54 0.454 (.007)                                                                        1.435                                                                             820100                          50/50 372   0.084                                                                             382   2327                                                                              4.94 1.95 0.347 (.002)                                                                        1.367                                                                             697600                          70/30 374   0.195                                                                             385   2431     0.93 0.595 (.005)                                                                        1.427                                                                             752900                          70/30 374   0.195                                                                             378   2559     0.93 0.595 (.005)                                                                        1.433                                                                             787600                          60/40 375   0.089                                                                             377   1985                                                                              5.36 1.33 0.418 (.003)                                                                        1.423                                                                             1103000                         60/40 375   0.089                                                                             374   2699                                                                              5.36 1.33 0.418 (.003)                                                                        1.408                                                                             815800                          50/50 374   0.201                                                                             367   2541                                                                              5.33 2.09 0.462 (.004)                              50/50 364   0.201                                                                             366   2670                                                                              5.33 2.09 0.462 (.004)                                                                        1.397                                                                             724300                          60/40 351   0.247                                                                             374   1948                                                                              4.62 1.03 0.466 (.009)                                                                        1.426                                                                             860000                          60/40 351   0.247                                                                             370   2326                                                                              4.62 1.03 0.466 (.009)                                                                        1.433                                                                             996700                          60/40 361   0.103                                                                             373   1605                                                                              5.53 1.57 0.387 (.005)                                                                        1.431                                                                             985400                          60/40 361   0.103                                                                             381   2221                                                                              5.53 1.57 0.387 (.005)                                                                        1.435                                                                             855800                          70/30 364   0.202                                                                             376   1837                                                                              5.25 1.50 0.429 (.010)                                                                        1.433                                                                             868300                          70/30 364   0.202                                                                             378   2376                                                                              5.25 1.50 0.429 (.010)                                                                        1.434                                                                             798100                          70/30 367   0.085                                                                             374   1593     1.48 0.378 (.002)                                                                        1.438                                                                             744200                          70/30 367   0.085                                                                             375   2145     1.48 0.378 (.002)                                                                        1.439                                                                             765000                          50/50 367   0.177                                                                             371   2393                                                                              5.08, 5.51                                                                         1.61 0.434 (.007)                                                                        1.408                                                                             889200                          50/50 367   0.177                                                                             371   3008                                                                              5.08, 5.51                                                                         1.61 0.434 (.007)                                                                        1.528                                                                             1029000                         50/50 366   0.085                                                                             370   2666     2.01 0.438 (.003)                                                                        1.405                                                                             922100                          50/50 366   0.085                                                                             369   2257     2.01 0.438 (.003)                                                                        1.383                                                                             922600                          __________________________________________________________________________

In Table I, the composite material is made from a polyvinyl chlorideknown as GEON 427 obtained from B.F. Goodrich Company. The wood fiber issawdust by-product of milling soft woods in the manufacture of woodwindows at Andersen Corporation, Bayport, Minn. The modulus for neatpolyvinyl chloride measured similarly to the composite materials isabout 430,000. The Youngs modulus is measured using an Instron Model450S (Series 9 software) automated materials testing system and uses anASTM method D-638. Specimens are milled to specifications of the testand are measured at 50% relative humidity, 73° F. with a cross set speedof 0.200 in./min.

Table II contains tensile modulus results from composite materials madeusing injection molding technology. The Table shows a substantialincrease in tensile modulus when compared to non-composite PVC.

We have found that the preferred pellet of the invention displays, in aformed structural member, a Youngs modulus of at least 500,000 andcommonly falls in the range greater than about 800,000, preferablybetween 800,000 and 2.0×10⁶. Further, the coefficient of thermalexpansion of the material is well matched to a compromising betweenaluminum, PVC and wood products and ranges from about 1.6 to 1.8×10⁻⁵inches per inch-fahrenheit degree. We believe that the superiorproperties of the structural members made from the composite or pelletof the invention are in large part due to the nature of the pellet setforth in the Table above. We believe the Table clearly shows that thepolyvinyl chloride and wood fiber can be combined at various proportionsunder a variety of temperature conditions to produce a regular pellet.The pellet then can be used in further extrusion processes to form auseful extruded structural member useful in the manufacture ofenvironmentally sensitive windows and doors. The composite is a superiorreplacement for wood because it has similar mechanical properties butattains a dimensional stability and resistance to rot, and insect damagenot attainable by wood products.

                  TABLE II                                                        ______________________________________                                        Injection Molded Samples                                                                   Tensile Modulus                                                                           Standard Deviation                                   Description  psi         psi                                                  ______________________________________                                        High Melt/   1,205,000   242,400                                              Large Pellet/40%                                                              PVC            488,800    28,370                                              High Melt/   1,232,000   133,300                                              Small Pellet/40%                                                              ______________________________________                                    

We claim:
 1. A composite thermoplastic pellet, capable of formation intoa structural profile or member, which pellet consists essentially of athermoplastic cylindrical extrudate having a width of about 1 to 5 mm, alength of about 1 to 10 mm, the thermoplastic pellet compositecomposition consisting essentially of:(a) about 40 to 70 wt. % athermoplastic polymer comprising vinyl chloride; and (b) about 30 to 50wt. % of wood fiber having a minimum aspect ratio of about 1.8 toprovide structural properties to the composite;wherein the polymer woodfiber are mixed at elevated temperature and pressure such that the fibercomprises less than about 8 wt-% water, the pellet comprises an intimateadmixture of wood fiber dispersed throughout the continuousthermoplastic polymer phase, the pellet is a recyclable thermoplasticand the composite has a Young's modulus of at least about 800,000 psi.2. The composition of claim 1 wherein the composite has a coefficient ofthermal expansion of less than 1.8·10⁻⁵ in/in-F°.
 3. The composition ofclaim 1 wherein the composite has a Young's modulus of greater thanabout 1,000,000 psi.
 4. The composition of claim 3 wherein the compositehas a coefficient of thermal expansion of less than 1.8·10⁻⁵ in/in-F°.5. The composition of claim 1 wherein the polymer occupies greater than50% of the interior volume of the fiber.
 6. The composition of claim 1wherein the polymer occupies greater than 70% of the interior volume ofthe fiber.
 7. The composition of claim 1 wherein the polymer comprises apolyvinyl chloride polymer alloy.
 8. The composition of claim 1 whereinthe wood fiber comprises a by-product of milling or sawing woodenmembers.
 9. The composition of claim 8 wherein the wood fiber comprisessawdust.
 10. The composition of claim 1 wherein the compositeadditionally comprises a compatibilizing agent.
 11. The composition ofclaim 1 wherein the polymer has a molecular weight (M_(w)) of about90,000±50,000.
 12. The composition of claim 1 wherein the polymercomprises a copolymer having a molecular weight of about 88,000±10,000.13. The composition of claim 1 wherein the wood fiber has a fiber widthof up to about 1.5 mm, a fiber length of up to about 10 mm and a aspectratio of about 2 to
 7. 14. The composition of claim 1 wherein the watercomprises about 0.01 to 5 wt-% of the pellet.
 15. The composition ofclaim 1 wherein the polymer comprises a polyvinyl chloride homopolymer.16. The composition of claim 1 wherein the wood fiber has an aspectratio of about
 6. 17. A polymer and wood fiber thermoplastic compositecomposition capable of formation into a structural profile or member,the thermoplastic extrudate composite composition consisting essentiallyof:(a) about 40 to 70 wt. % a thermoplastic polymer comprising vinylchloride; and (b) about 30 to 50 wt. % of wood fiber having a minimumaspect ratio of about 1.8 to provide structural properties to thecomposite;wherein the polymer wood fiber are mixed at elevatedtemperature and pressure such that the fiber comprises less than about 8wt-% water, the pellet comprises an intimate admixture of wood fiberdispersed throughout the continuous thermoplastic polymer phase, thepellet is a recyclable thermoplastic and the composite has a Young'smodulus of at least about 800,000 psi.
 18. The composition of claim 17wherein the composite has a coefficient of thermal expansion of lessthan 1.8·10⁻⁵ in/in-F°.
 19. The composition of claim 17 wherein thecomposite has a Young's modulus of greater than about 1,000,000 psi. 20.The composition of claim 19 wherein the composite has a coefficient ofthermal expansion of less than 1.8·10⁻⁵ in/in-F°.
 21. The composition ofclaim 17 wherein the polymer comprises a polyvinyl chloride copolymer.22. The composition of claim 17 wherein the polymer comprises apolyvinyl chloride polymer alloy.
 23. The composition of claim 17wherein the wood fiber comprises a by-product of milling or sawingwooden members.
 24. The composition of claim 17 wherein the wood fibercomprises sawdust.
 25. The composition of claim 17 wherein thecomposition additionally comprises a compatibilizing agent.
 26. Thecomposition of claim 17 wherein the polymer has a molecular weight(M_(w)) of about 95,000±50,000.
 27. The composition of claim 17 whereinthe polymer comprises a copolymer has a molecular weight (M_(w)) ofabout 88,000±10,000.
 28. The composition of claim 17 wherein the woodfiber has a fiber width of up to about 1.5 mm, a fiber length of up toabout 10 mm and a aspect ratio of about 2 to
 7. 29. The composition ofclaim 17 wherein the water comprises about 0.01 to 5 wt-% of the pellet.30. The composition of claim 17 wherein the wood fiber has an aspectratio of about
 6. 31. A polymer and wood fiber composite capable offormation into a structural member, which composite comprises acylindrical linear extrudate having a radius of about 1 to 5 mm; thelinear extrudate composition comprising:(a) about 45 to 70 wt-% of apolymer comprising vinyl chloride; (b) about 30 to 50 wt-% of wood fiberhaving a minimum thickness of 1 mm and a minimum length of 3 mm and anaspect ratio of about 1.8; (c) less than about 8 wt-% water; and whereinthe wood fiber is dispersed throughout a continuous polymer phase. 32.The composition of claim 31 wherein the polymer comprises a polyvinylchloride homopolymer.
 33. The composition of claim 31 wherein thepolymer comprises a polyvinyl chloride copolymer.
 34. The composition ofclaim 33 wherein the copolymer has a molecular weight of about88,000±10,000.
 35. The composition of claim 31 wherein the wood fibercomprises a by-product of milling or sawing wooden members.
 36. Thecomposition of claim 35 wherein the wood fiber comprises sawdust. 37.The composition of claim 35 wherein the polymer has a molecular weightof about 95,000±50,000.
 38. The composition of claim 31 wherein thecomposition additionally comprises a compatiblizing agent.
 39. Thecomposition of claim 31 wherein the wood fiber has a fiber width ofabout 0.3 to 1.5 mm, a fiber length of about 1 to 10 mm and a aspectratio of about 2 to
 7. 40. The composition of claim 31 wherein the watercomprises about 0.01 to 5 wt-% of the pellet.
 41. A compositecomposition, capable of extrusion into a dimensionally stable structuralmember, which composition comprises:(a) about 40 to 70 wt-% of a polymercomprising vinyl chloride; (b) about 30 to 50 wt-% of wood fiber havinga minimum width of 0.3 mm and a minimum length of 1 mm and an aspectratio of greater than about 1.8; (c) about 0.01 to 25 wt-% of anintentionally recycled impurity comprising thermoplastic polymer, anadhesive, a paint, a thermoplastic resin or mixtures thereof; and (d)less than about 10 wt-% water;wherein the wood fiber is dispersedthroughout a continuous polymer phase.
 42. The composition of claim 41wherein the polymer comprises a polyvinyl chloride homopolymer.
 43. Thecomposition of claim 42 wherein the polymer has a molecular weight ofabout 95,000±50,000.
 44. The composition of claim 41 wherein the polymercomprises a polyvinyl chloride copolymer.
 45. The composition of claim44 wherein the copolymer has a molecular weight of about 88,000±10,000.46. The composition of claim 41 wherein the wood fiber comprises aby-product of milling or sawing wooden members.
 47. The composition ofclaim 41 wherein the wood fiber comprises sawdust.
 48. The compositionof claim 41 wherein the wood fiber has a fiber width of about 0.3 to 1.5mm, a fiber length of about 1 to 10 mm and a aspect ratio of about 2 to7.
 49. The composition of claim 41 wherein the concentration of watercomprises about 0.01 to 3.5 wt-% of the composition.
 50. The compositionof claim 41 wherein the intentionally recycled impurity comprises about0 to 5 wt-% of a hot melt adhesive composition, about 0 to 5 wt-% of apaint and about 0.01 to 15 wt-% of recycled polyvinyl chloride cladding,wherein the cladding comprises about 9.0 to 99.9 wt-% polyvinyl chlorideand about 0.01 to 2 wt-% of a pigment.